1. Field of the Invention
The present invention generally relates to reducing harmonic interference from local oscillators in a dual conversion receiver.
2. Background Art
Television signals are transmitted at radio frequencies (RF) using terrestrial, cable, or satellite transmission schemes. Terrestrial and cable TV signals are typically transmitted at frequencies of approximately 57 to 860 MHZ, with 6 MHZ channel spacings in the United States and 8 MHz channel spacing in Europe. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.
Regardless of the transmission scheme, a tuner is utilized to select and down-convert a desired channel from the TV signal to an intermediate frequency (IF) signal or a baseband signal, which is suitable for processing. The signal may be a television picture in analog or digital format, or digital data (e.g. for a cable modem).
To achieve a high level of image rejection, some TV tuners utilize a dual-conversion architecture having two mixers and two IF filters. The first mixer up-converts the received RF signal to a first IF frequency (e.g. 1220 MHZ) that is fixed above the RF signal band of the incoming TV signal, using a variable local oscillator (LO) signal. An IF filter, centered at the first IF, selects the channel of interest and provides the image rejection to prevent signal interference. The second mixer then down-converts the first IF to a lower frequency second IF, using a second fixed or variable frequency LO signal. The second IF is typically at 36 or 44 MHZ and is selected by a second IF filter with high selectivity that removes any undesired signal interference. Channel selection is realized by adjusting the first LO signal so that the desired channel is up-converted into the passband of the SAW filter, and is then down-converted to the passband of the second IF filter by the second mixer.
All practical local oscillators generate harmonic frequencies, in additional to the intended fundamental frequency. For instance, if the intended fundamental frequency of the local oscillator is 1000 MHz, the local oscillator will also generate frequencies at 2000 MHz, 3000 MHz, and so on, at increasingly reduced amplitudes. One problem with dual conversion tuners, is that harmonics of the first and second local oscillators can mix, for example in the second mixer stage, and fall in the passband of the baseband SAW filter. When this occurs, the unwanted harmonics can interfere with the down-converted channel.
What is needed is a method or apparatus for preventing harmonic interference in the dual conversion receiver.